i said "your charging a battery with a battery so there will be a good bit of lost potential on long charging cycles.
the longer you charge the more heat is generated, heat degrades battery life"
the battery doesnt have to reach detrimental temperatures to lower the stamina of a battery.
if you take a battery thats 70deg and one thats 100deg, you will get more life out of the 70deg battery every time.
elevated temps lower the over all value of a battery, its a known fact.
You are correct, and I didn't mean for my post to sound like I was discounting your comment. Perhaps you missed my comment further on... "The previous post was a good explanation.", referring you your post.
Certainly heat is detrimental to virtually all electronics, and Lithium based batteries do in fact lose certain increasing percentages of their ability to hold charge over time, even under normal use. Also, heat will essentially accelerate that process. Still, for practical purposes the potential losses would be miniscule from the durations of exposure to heat from the infrequent cycling of charges.
Also, the charging circuitry monitors the temperature of the battery and knows what levels are detrimental, so it shuts down the charging process (or more accurately limits it) while monitoring the temperature until the battery temperature reverts to a "safe temperature range" at which point the charging circuitry resumes the charging at whatever rate it was at based on where in the charging process it was at that time. However to your point, at least one study (performed by Battery University) has shown that
constant exposure to excessive heat (i.e. 24/7/365) will produce capacity losses ranging from 10% to 15% of initial rated capacity for temperatures elevated to 40C (104F) for one year versus the same period at 25C (77F). In other words, it's not so much that the temperature goes up, but more so for how long it remains up that is the catalyst for battery degradation.
I believe that the infrequent and short durations of potentially elevated battery temperatures SallyC will experience using the battery to battery charging process as opposed to the other options is not likely to have any appreciable impact on the useable lifespan of her battery, and the fact that she is charging more frequently (a known good) rather than less may actually offset any temperature-based loss impact as well. Interestingly enough, keeping the batteries at full charge rather than partial charge for extended periods of time also has a similar degrading effect as the chart shows (again a testamentary to short charging cycles for anyone listening), and a combination of heat and a full charge has an even greater detrimental effect.
I do have one comment, that is...it's not the length of the charging as you mention, so much as the RATE of charge that causes heat. In other words, it is entirely possible to charge a battery over even days rather than hours and produce very little increase in temperature. You simply need to reduce the voltage level of the charging rate or limit the current. On the other hand, even a short charging cycle (i.e. 10 minutes) from a "rapid charger" (which typically push higher voltages and have greater current limits), can elevate the battery's temperatures quickly and potentially into the ranges we've been discussing here. Even a car accessory rapid charger will cause the battery on the phone to heat up quicker than the wall-wart charger (charging block) that ships with the phone.
You did also mention another factor for prolonging battery life than I have made many posts regarding...the length of charging cycles and the recommendation for refraining from cycling between full discharges and full recharges. To quote you; "therefore keep the charge cycles short as possible for longest life." The fact is that Lithium based batteries will yield much longer service life if charged more frequently and for shorter "top off" or "interim boost" charges rather than 0% to 100%. I am a huge advocate of plugging in whenever there's ability to do so - whether it be a portable charging pack as SallyC referred to, a wall-block that you carry with you, a car accessory adapter, a docking station or car navigation dock, or even just the USB port on a desktop PC, for instance. No matter what your source of choice, your battery will last longer if it is more frequently replenished in shorter bursts rather than long overnight charges with no suplemental charging through the day.
I have quoted Battery University and referenced them many times before in links. Here's a great link that affirms what I am saying and also provides reference to the temperature study.
How to Prolong Lithium-based Batteries - Battery University
To recap, I wasn't disagreeing with you, more the contrary - but I was trying to look at the situation from a more practical, real-world point of view. The article mentioned above also discusses using those wireless charging mats and suggests that they may also add to the degradation over time, for the same reason you mentioned...increased heat. Again, I have to question though, how much real capacity loss would be incurred over time, and would it be notable or essentially undetectable under normal use. I think that in a real-world scenario, the lifespan of the battery would be impacted minimally and it's more likely that the phone would be replaced before the effect from the increased temperature would be notable. Don't forget, there's also the normal decreasing capacity that happens over time and also by the number, starting and ending charge levels and durations of charging cycles, so without a true study, no one will know for sure.
